This invention relates data communications devices and systems, and in particular to a system for selectively interconnecting multiple low-cost local communication buses to a larger, global communication bus.
Data communication systems have been developed having a wide variety of capabilities and costs. Different configurations are available for a wide variety of applications in the fields of data communication and control. Some systems have been developed having extensive capabilities in terms of performance and the ability to be expanded to very large networks of devices to be controlled or communicated with. Unfortunately, such systems often are very expensive, and are therefore better suited to applications where their performance advantages are essential. Other system have been developed in the prior art for data communications and control over a small, essentially local collection of devices. Such systems can provide good performance for their intended tasks, while avoiding much of the high cost and complexity of large systems.
An example of such a low cost system is the time division multiplexed (TDM) single pair, or half-duplex communication buses and devices which have been developed in the field of building management systems. Such systems may use one or more such local buses to monitor and control burner systems and related components in a heating system. Such systems are small and essentially local in nature, and support only a limited number of device types. Each device type has a set time slot, from a starting marker, to broadcast data about itself onto the bus. Other devices, such as displays or controllers are designed to listen for and use the information from certain types of devices, occurring at certain time slots. For example, a burner control may transmit bursts of data in its time slot giving current operating parameters, and a display module is designed to listen in that time slot, then display the data. The TDM approach is useful in keeping costs and complexity down, because it avoids the complexities of addressing schemes, token communication protocols, polling, and the like. However, a shortcoming of such systems is limited expandability. Because it is based on the premise of each device type having a fixed time slot, multiple such devices cannot be supported on the same bus.
The prior art has attempted to address this limitation by providing a control or communication device as one of the time-slot device types in the local bus, then connecting the control/communication device into a larger communication system, by providing a data telephone line or a computer card interface. While successful, such prior art systems also run into limitations in that it becomes expensive to provide phone lines or computer interface cards for more than about six or so local buses. This may work for medium-sized installations, but is problematic for large installations such as factories, where there may be perhaps fifty or more burners to monitor. This would lead to a proliferation of computers or phone lines, and the attendant cost and complexity. The only other alternative would be a fully addressable network for all devices, which would also entail significant costs, and would give up the advantages of the simple and established TDM bus system.
The present invention provides a method and devices for allowing multiple local simple communication buses to be selectively and separately connected as part of a larger, global communication bus. This interconnection is done transparently to the local buses, which may continue to operate on a simple and low cost protocol. This permits the use of readily available and low cost monitoring/control devices, thus preserving system economy.
According to one aspect of the invention, a communication system is provided, including a plurality of local bus systems, each having a communication bus and a plurality of devices connected thereto and operative to communicate on the local communication bus. A plurality of switch devices are provided, each connectable to one of the local communication buses, and to a global communication bus. A controller is connected to the global communication bus and operative to transmitting signals thereon addressed to multiple or selected individual switch devices. A selected switch device is operative in response thereto to connect its local communication bus to the global communication bus for transmission of signals therebetween.
According to another aspect of the invention a fault polling mode of operation is provided, used where at least some of the devices on the local communication buses generate fault codes in response to detected fault conditions, wherein the communication system controller is operative in a fault polling mode to transmit a fault polling command to the switch devices, and wherein the switch devices are operative to check their local bus systems for faults and to provide an indication thereof to the controller.
According to another aspect of the invention used where the local communication bus systems operate on a local synchronous protocol, and wherein the controller addresses the switch devices in an asynchronous mode of operation, then communicates with a selected local bus in its synchronous protocol.